Design and Materialization of Ionic Liquids Based on an Understanding of Their Fundamental Properties

Abstract

Ionic liquids (ILs) are defined as salts that have melting points lower than 100°C. Most are organic salts, and these may be designed and tailored to have suitable properties. ILs are recognized as a third group of solvents (and electrolytes), after water and organic solvents. They are characterized by their unique properties such as nonvolatilities, high thermal stabilities, and high ionic conductivities. In this article, our work on the design and preparation of ILs is briefly reviewed. The concept of ionicity is proposed as a metric to characterize the basic nature (dissociativity) of ILs, which is affected by the Lewis acidity/basicity of cations/anions (i.e., coulombic interactions), the directionality of interactions between cations and anions, and van der Waals interactions between ions. The ionicity of ILs is dominated by a subtle balance between coulombic and van der Waals interactions, which clearly discriminates ILs from conventional high-temperature inorganic molten salts. Here, the design of protic ILs for fuel cell electrolytes, electron-transporting ILs for dye-sensitized solar cell electrolytes, and Li + -conducting solvate ILs for lithium battery electrolytes are discussed based on an understanding of the fundamental properties of ILs. Furthermore, the combination of ILs with polymers and colloidal particles affords intriguing quasi-solid materials (ion gels), and the ion transport in these gels is decoupled from the mechanical relaxation time of these materials, yielding new solid electrolytes. The possibility of temperature and photo-sensitive solubility dependence of polymers in ILs allows the creation of stimuli-responsive materials. Finally, protic ILs/protic salts are demonstrated to be good precursors for N-doped carbon materials.

Self‐Healing Micellar Ion Gels Based on Multiple Hydrogen Bonding

“…Ion gels, which are defined as macromolecular networks filled with ionic liquids (ILs), have been attracting interest as promising soft solid electrolytes for application to flexible electronic devices. Relative to hydrogels and organogels, ion gels exhibit unique characteristics such as high ionic conductivity, high chemical and thermal stability, nonvolatility, and nonflammability, all of which can be attributed to the intrinsic properties of ILs . From a practical viewpoint, solution‐processable ion gels that can be directly printed onto substrates are desirable for the fabrication of low‐cost flexible electronic devices .…”

Classifying Electrolyte Solutions by Comparing Charge and Mass Transport

“…[19] Although several important models for the transport properties of concentrated electrolytes were published over the last decades, [20][21][22][23][24] there are still controversial viewpoints in the literature as to whether highly concentrated electrolytes exhibit a low degree of dissociation ("low ionicity") and can thus be classified as "weak" or "diluted" electrolytes. [25][26][27][28][29][30] In this Outlook, we show that a more general classification of binary electrolytes (one salt, one solvent), encompassing also highly concentrated solutions, can be achieved by taking into account both charge and mass transport properties. We use the Onsager transport formalism in combination with linear response theory to define a molar mass transport coefficient, Λ mass , which is the mass transport analog of the molar ionic conductivity, Λ charge .…”

Novel Ionic Conducting Composite Membrane Based on Polymerizable Ionic Liquids

“…Up to now, several types of polymers have been tested in medium-and high-temperature PEMFCs, namely polyimides, polysulfones, polybenzoxazoles, poly(ether ether ketone)s, polybenzimidazoles, polyoxadiazoles, and nanostructured organic-inorganic composites [5][6][7][8][9][10][11][12][13]. The H 3 PO 4 doping of these polymers is one of the easiest and most often-used approaches to design proton-conductive membranes.…”